For applications which require more than one LED, the LEDs are commonly connected in series as a so-called “LED string”. The current through the LED string is controlled by a linear current source or by a switching regulator.
Switching regulators, such as DC-DC converters, require an inductor, and typically two capacitors. Each of these may be bulky, and contribute to the cost of the regulator. Conversely, linear current sources are generally compact and relatively inexpensive; however linear current sources generally can only handle a large input voltage range at the expense of low efficiency and large dissipation. This may prohibit their use unless the supply voltage range is limited.